Recently a digital loop carrier service is being introduced mainly in North America for subscribers being remotely located from switching equipment. The subscribers are connected to the switching equipment through multiplexed optical transmission lines.
In FIG. 1, a configuration example of a UDLC (Universal Digital Loop Carrier) system is shown. A DS0 level analog output from an analog switch 1 is converted into a digital signal by means of digital loop carrier equipment 2, and is converted further to an optical multiplexed signal 4, such as OC3/OC12, by an add-drop multiplexer (ADM) 3. The signal is then forwarded to a remote terminal station 5 which accommodates a terminal located in a carrier service area 6.
Also, a DS0 analog output from analog switch 1 is converted into an optical multiplexed signal 8 through an access carrier transport system (ACTR) 7 having a dynamic cross connect function. The signal is then forwarded to a remote terminal station 9 which accommodates a terminal located in a carrier service area 10.
In FIG. 2, there are shown digital switches (TR008 compliant and TR303 compliant), which output DS1 signals. Digital switches 11, 12 are connected oppositely to remote terminal stations 13, 14 of access carrier transport systems (ACTR), constituting an Integrated Digital Loop Carrier (IDLC) system.
Here, a digital switch (TR008) 11 is one type of switching equipment which transmits/receives control signals through a service channel within each subscriber line. A digital switch (TR303) 12 is another type of switching equipment which uses a different path for control signal transmission through a dynamic cross connect.
The systems shown in FIGS. 1, 2 are described below in more detail. Conventional analog switches 1A–1C have the identical interface, irrespective of the switching equipment venders, conforming to the identical interface standard (TR-008). Regarding ISDN in particular, alarm notification and fault locating method is specified by TR-397 standard.
In addition, a central office terminal (COT) 20 and a remote terminal station (RT) (or, subscriber transmission equipment) 21 being located between switches 1A–1C and subscriber terminal equipment 22 constitute the UDLC structure shown in FIG. 1, which is so called ‘invisible existence’.
Therefore, in designing both central office terminal (COT) 20 and remote terminal (RT) 21, it was not necessary to take switching equipment venders into consideration. Basically it is possible to transmit signals between switches 1A–1C and subscriber terminals 22 without modification (i.e. transparently).
More specifically, in TR-008 mode, it is possible to send a common m-bit and a control path message (eoc: embedded operation channel) using the identical specification without regard to the switching equipment venders in the event of an ISDN alarm condition.
In table 1 shown in FIG. 4, there is illustrated ISDN alarm notification command and the values thereof (TR-008) applicable to every switching equipment vender. For example, in the case of CH card extraction listed in item 1, on detection of an interface LUNT in central office terminal (COT) 2 being not installed, m-bit=‘0111 1110 111’b is sent to switch 1.
Each bit is named (from left to right) as act, ps1, ps2, ntm, cso, m46, sai, nib, m51, m52 and m61. In the above case, act and nib are ‘0’, while the others are ‘1’.
Also, in the case of item 1, eoc message having addr.=1 and msg=‘78’ is sent to switch 1. Here, addr.=1 denotes LUNT (COT side), addr.=2 denotes LULT (RT side), and addr.=0 denotes NT1, respectively. According to the indication of msg=‘78’ which denotes ‘loss of synchronization’, it is to be understood that an alarm condition occurs in interface LULT corresponding to addr.=2, causing an alarm sent from addr.=1 to switch 1.
Similarly, in the case of loss of synchronization at reference point U shown in item 2, m-bit is also sent as in the case of item 1, while eoc address is changed to ‘2’.
In the case of NT1 power off shown in item 3, m-bit and control path (eoc) are sent, similarly to item 2, because the state is moved to the loss of synchronization at point U which is the same as item 2.
In the case of loss of synchronization at reference point T shown in item 4, both act bit and sai bit are ‘0’ and the remainder bits are ‘1’. Even the synchronization is lost at point T, the synchronization to network terminal NT1 is still in established condition. Therefore, control path (eoc) having ‘7’, which denotes a broadcast address, and a message having ‘FF’, which denotes ‘return to normal’ are sent.